Potentiometric determination of relative basicities of aliphatic amines in pyridine

Potentiometric titrations of methyl-, ethyl-, n-propyl-, n-butylamines and ammonia were carried out in pyridine using a standardized tetrabutylammonium hydroxide solution. The relative basicities were found in all cases to be in the order RNH2 > R2NH > R3N, R being an alkyl group. Basicity of any amine was assumed to be the sum of the inductive contribution of alkyl group(s) and a contribution from pyridine solvent through hydrogen bonding. On this assumption, the experimental basicity order was rationalized. The order in pyridine is different from the order of gas phase basicities.